1. Field of the Invention
This invention relates to power amplifiers and more specifically to a Class A high fidelity audio amplifier having high efficiency.
2. Description of the Prior Art
Cross-over distortion is a well known disadvantage of "Class AB" push-pull amplifiers caused by abrupt non-linearities in the transfer function. This disadvantage is eliminated by using "Class A" operation in which the output transistors are always operated in their linear operating region. A disadvantage of Class A amplifiers is their low efficiency, since the bias current is usually at least half the peak output current in a push-pull amplifier.
A diagram of the output stage of a prior art class AB push-pull amplifier is shown in FIG. 1. The output stage is shown in a symmetrical configuration. Transistors Q1 and Q2 control current to the output node D. The current is supplied from two voltage sources: a first source V1 having a positive voltage applied to the collector of Q1 (node B) and a second source V2 having a negative voltage applied to the collector of Q2 (node C). The emitters of Q1 and Q2 are both connected to output node D through emitter resistors R1 and R2. Biasing of Q1 and Q2 is supplied by voltage sources V3 and V4. V3 keeps the base of Q1 at a constant positive voltage relative to input node A. Likewise, V4 keeps the base of Q2 at a constant negative voltage relative to input node A. The magnitudes of V3 and V4 are equal, the magnitudes of Vbe1 (base-emitter voltage of Q1) and Vbe2 are equal, and the values of R1 and R2 are equal.
In such a class AB push-pull amplifier, both output transistors Q1, Q2 are biased in their linear active region for small values of output current. In other words, for small signals both transistors are always conducting and operating as a class A amplifier. For large signals output transistors Q1, Q2 will alternately be driven into cutoff. A discontinuity is created in the transfer function of the amplifier at the point of cutoff and this is a form of undesirable crossover distortion.
During no signal conditions, Vout=Vin, Ie1=-Ie2, and Iout=Ie1+Ie2=0. Since Vout=Vin, voltage V3 is dropped across Vbe1 and R1 and the voltage V4 is dropped across Vbe2 and R2. If Vin is raised, the voltage drop across R1 will increase and the voltage drop across R2 will decrease. Current Ie1 will increase and current Ie2 will decrease. The difference in Ie1 and Ie2 will flow out of node D into the load impedance. As Vin continues to rise in the positive direction, Ie1 will increase and Ie2 will decrease until it becomes zero and transistor Q2 is cutoff. Since Q2 cannot reverse the polarity of its emitter current, Ie1 will continue to increase as Ie2 remains at zero. With Q2 cutoff, output current Iout is equal to the emitter current of Q1; Iout=Ie1. If Vin is lowered, Ie1 will decrease until Q1 is cutoff and output current Iout will be equal to the emitter current of Q2; thus Iout=Ie2.
FIG. 2 shows Ie1, Ie2, and Iout as a function of Vin-Vout. At points E and F on the Vin-Vout axis one of the output transistors Q1, Q2 goes into cutoff and the slope of Iout changes. The change in slope of the Iout curve represents a change in the output resistance of the amplifier. If both transistors Q1, Q2 are in the linear active region, the output resistance can be approximated by the parallel combination of R1 and R2. If one of the transistors Q1, Q2 is cutoff, the output resistance increases to the value of just one emitter resistor, either R1 or R2.
One known method to reduce the discontinuity of the Iout curve shown in FIG. 2 and to maintain the efficiency of class AB operation is to prevent either output transistor Q1, Q2 from going into cutoff, yet allowing a low quiescent current during no signal conditions.
Several disclosures apply this principle to a complementary push-pull emitter follower output stage of the type shown in FIG. 1. U.S. Pat. Nos. 4,250,323, 4,558,288, 4,595,883 to Nakayama each disclose use of two auxiliary amplifiers. Each auxiliary amplifier prevents an output transistor from going into cutoff, but by their operation the auxiliary amplifiers themselves include transistors which go into cutoff and effectively disconnect the output transistors from the input signal.
Another technique disclosed by Kawanabe (U.S. Pat. No. 4,215,318) uses two auxiliary amplifiers to adjust instantaneously the bias across both transistors to prevent either from going into cutoff. Both of these auxiliary amplifiers include transistors that by their operation go into cutoff.
Okabe (U.S. Pat. No. 4,274,059) applies feedback to one output transistor at a time. This technique requires diodes which must switch on and off.
Pass (U.S. Pat. No. 3,995,228) also applies feedback to one output transistor at a time. This technique also requires diodes which must switch on and off.